The invention relates to a picture display of the rear surface projection type, and especially to a picture display of the rear surface projection type in which a light flux modulated by a picture is projected on a rear surface of a transparent screen, and the projected picture is enjoyed by a user from the front side of the transparent screen.
Some of the picture displays of the rear surface projection type developed in recent years are provided with built-in picture-forming units, such as liquid crystal light valves or digital mirror devices. In the aforementioned apparatus, the light flux outputted from the light source is modulated by the picture generated in the picture-forming unit, shot out from the projector, and projected on the rear surface of the transparent screen. The picture projected on the screen is enjoyed by a user from the front side of the transparent screen.
However, if the picture display of the rear surface projection type is used in such a condition that the optical axis of the projector is intersected with the transparent screen perpendicularly, the distance between the projector and the transparent screen cannot be shortened, and it becomes difficult to make the picture display thin. In order to solve the aforementioned problem, it is the general trend that the angle of view of a projection optical system of the projector is widened, the projector is shifted down to a position lower the transparent screen, and the light flux shot out from the projector is projected on the rear surface of the transparent screen obliquely.
However, in the aforementioned picture display of the rear surface projection type, since the incidence angle of the light flux incident on the transparent screen becomes large, and thereby the greater part of the incident light flux is reflected at the rear surface of the transparent screen and cannot be transmitted thereinto, the brightness of the picture becomes low when viewed from he front side of the screen, and the quality of the picture is degraded.
Accordingly, it is an object of the invention to provide a picture display of the rear surface projection type in which an incident light flux is efficiently transmitted into a transparent screen even when an incidence angle of a light flux increases because of the widened angle of view of a projection optical system, and the brightness and the quality of a picture are certainly prevented from being degraded because of reflection of an incident light flux at a rear surface of a transparent screen, and a picture with the excellent quality can be enjoyed.
According to the feature of the invention, a picture display of the rear surface projection type comprises:
a projector for shooting out a light flux modulated by a picture,
a transparent screen, on a rear surface of which the light flux shot out from the projector is projected, and
a sawlike prismatic surface which is formed on the rear surface of the transparent screen, and provided with plural edges shaped into concentric circles centering around a central point situated outside the transparent screen,
wherein an optical axis of the projector passes through the central point, and
a ray of light incident on a first face looking downward and neighboring with each of the plural edges is efficiently transmitted into the transparent screen, and the ray of light transmitted through the first face is totally reflected by a second face looking upward and neighboring with the same edge to a front surface of the transparent screen, in case that a angle formed by the ray of light incident on the first face and the optical axis of the projector is greater than 40xc2x0 and less than 90xc2x0.
In the picture display of the rear surface projection type according to the invention, when the angle formed by the optical axis of the projector and the ray of light incident on the first face is greater than 40xc2x0 and less than 90xc2x0, the ray of light incident on the first face looking downward and neighboring with each edge formed on the sawlike prismatic surface is efficiently transmitted into the transparent screen, and totally reflected by the second face looking upward and neighboring with the same edge to the front surface of the transparent screen. Accordingly, even in case that the angle of view of the projection optical system is widened, and thereby the incidence angle of the light flux incident on the transparent screen increases, the incident light flux is efficiently transmitted into the transparent screen, and can be totally reflected to the front surface thereof. Accordingly, the brightness and the quality of the picture are certainly prevented from being degraded, and the picture with the excellent quality can be enjoyed by the user.
In order to achieve the aforementioned object of the invention, an angle a xcex12 formed by the first face and a line perpendicular to the optical axis of the projector is given by a following equation that
tan xcex12=└n2 sin {sinxe2x88x921((n3/n2)sin xcex82+n1 sin xcex81)+2xcex11}+n1 sin xcex81┘/[n1 cos xcex81xe2x88x92n2 cos {sinxe2x88x921((n3/n2)sin xcex82+n1 sin xcex81)+2xcex11}],
wherein a refractive index of the first medium brought into contact with the sawlike prismatic surface of the transparent screen is denoted by n1, a refractive index of the second medium forming the transparent screen is denoted by n2, a refractive index of the third medium brought into contact with a front surface of the transparent screen is denoted by n3, an angle formed by the ray of light incident on the first face and the optical axis of the projector is denoted by xcex81, a refraction angle of a ray of light shot out from the front surface of the transparent screen is denoted by xcex82, and an angle formed by the first and second faces is denoted by xcex11.
When the refractive indexes n1 to n3 the angle xcex81 formed by the optical axis of the projector and the ray of light incident on the first face looking downward, etc. are given in the process for fabricating the sawlike prismatic surface, the angle xcex12 (the Fresnel angle) are determined from the above equation, and the structure of the sawlike prismatic surface in which various incommoditeis, such as the surface reflection of light etc., can be avoided is uniquely determined.
In order to achieve the aforementioned object of the invention, a transmission efficiency xcex7 of the ray of light incident on the sawlike prismatic surface is given by a following equation that
xcex7=sin xcex12 cos xcex12 {tan(90xc2x0xe2x88x92xcex12)+tan xcex81}{(1/tan xcex11)xe2x88x92tan xcex81b},
wherein xcex81b is a refraction angle of the ray of light incident on the first face looking downward of the sawlike prismatic surface. If xcex11 such that xcex7 is greater that 1 is determined from the above equation, the picture with the excellent quality can be obtained without attenuating its brightness.
Moreover, it is desirable to form a light absorption layer for absorbing an external light penetrating into the transparent screen through a front surface thereof on an external surface of each of the second faces looking upward. For example, if the external light penetrates into the transparent screen through the front surface thereof toward the rear side, there arises a possibility that the external light will be scattered diffusedly by inner surfaces of a casing for accommodating the transparent screen, and undesired images (ghosts) will be formed at irregular positions on the picture. However, if the aforementioned structure is adopted, since the external light is absorbed by the light absorption layer and not transmitted toward the rear side, the undesired images can be certainly prevented from being formed.